This invention relates to a structure for mounting a cover for an automatic focus detecting device using an automatic focus detecting element such as a self-running type photo-electric element (hereinafter referred to as "an image sensor", when applicable) in a single-lens reflex camera.
First, one example of a focus detecting principle, i.e. a so-called "contrast method" will be described. The optical image of an object to be photographed, which is formed by a photographing lens, has a maximum difference between the light and shade portions thereof, i.e. the image has a maximum contrast, when the lens is focused on the object. This phenomenon is due to the fact that the power spectrum of the spatial frequencies of the object becomes maximum at the time of focusing. A variety of devices for detecting a focusing position by utilization of this phenomenon have been proposed in the art. One example of the automatic focus detecting element in the automatic focus detecting deivce utilizing the contrast of an object to be photographed is a device in which an image sensor is used to electrically scan the power spectral distribution of an object's image into a time-series signal, and the time-series signal is electrically processed to obtain the contrast data of the object's image dynamically.
The image sensor is constituted by a plurality of minute photoelectric elements and a scanning circuit, and image sensors may be either MOS-FET type or CCD type, both of which are commercially available, depending on the types of scanning circuits employed.
An automatic focus detecting device using an image sensor can be provided merely by using a simple mechanism comprising an image sensor and an electrical circuit, with the image sensor set at a position equivalent to the film surface. Therefore, an automatic focus detecting device utilizing an image sensor is advantageous in that it is considerably compact. In arranging the automatic focus detecting device using an image sensor, a light beam forming the image of an object must be introduced to the finder system and the focus detection system simultaneously. For this purpose, for instance, a method is employed in which the light beam is split into two parts by a half mirror and a total reflection mirror so that one of the two parts is used to form the object's image on the focusing screen in the finder system and the other is used to form the object's image on the image sensor in the focus detection system. Such an arrangement is shown, for example, in U.S. Pat. No. 4,180,309.
In this type of device, it is necessary that the mirrors mentioned above are retracted from the optical path of the photographing image to prevent vignetting on the film surface during the photographing operation. Steps should also be taken to prevent the occurrence of ghosts on the film surface due to unwanted reflection from the focusing screen in the finder system and from the protective plate such as a glass plate of the image sensor in the focus detection system. It is well known in the art that, in the case of the focusing screen, the mirror holding frame is lifted at the photographing time to cover the focusing screen to thereby prevent the reflection from the focusing screen and to prevent the leakage of light from the finder side to the film surface. It goes without saying that the unwanted reflection from the protective plate of the image sensor must also be prevented since it is also harmful to photographing as was described above.
On the other hand, if the image of an object on the focusing screen in the finder system is not completely coincident with that on the image finder in the focus detection system, then the automatic focus detecting device cannot be used. Therefore, focus matching is needed for the finder system and the focus detection system, and it is desirable that the focus matching can be readily achieved even after the camera has been assembled. Further, if dust or the like is stuck onto the protective plate of the image sensor, it may cause some light blockage and, accordingly, the sensor cannot operate satisfactorily. Thus, it is also necessary to keep the surface of the protective plate of the sensor clean at all times.